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This patch attempts to make the rules for data allocation in the packet explicit, understandable, and easy to verify. The constructor that copies a packet is extended with an additional flag "alloc_data" to enable the call site to explicitly say whether the newly created packet is short-lived (a zero-time snoop), or has an unknown life-time and therefore should allocate its own data (or copy a static pointer in the case of static data). The tricky case is the static data. In essence this is a copy-avoidance scheme where the original source of the request (DMA, CPU etc) does not ask the memory system to return data as part of the packet, but instead provides a pointer, and then the memory system carries this pointer around, and copies the appropriate data to the location itself. Thus any derived packet actually never copies any data. As the original source does not copy any data from the response packet when arriving back at the source, we must maintain the copy of the original pointer to not break the system. We might want to revisit this one day and pay the price for a few extra memcpy invocations. All in all this patch should make it easier to grok what is going on in the memory system and how data is actually copied (or not).
This is the gem5 simulator. The main website can be found at http://www.gem5.org A good starting point is http://www.gem5.org/Introduction, and for more information about building the simulator and getting started please see http://www.gem5.org/Documentation and http://www.gem5.org/Tutorials. To build gem5, you will need the following software: g++ or clang, Python (gem5 links in the Python interpreter), SCons, SWIG, zlib, m4, and lastly protobuf if you want trace capture and playback support. Please see http://www.gem5.org/Dependencies for more details concerning the minimum versions of the aforementioned tools. Once you have all dependencies resolved, type 'scons build/<ARCH>/gem5.opt' where ARCH is one of ALPHA, ARM, NULL, MIPS, POWER, SPARC, or X86. This will build an optimized version of the gem5 binary (gem5.opt) for the the specified architecture. See http://www.gem5.org/Build_System for more details and options. With the simulator built, have a look at http://www.gem5.org/Running_gem5 for more information on how to use gem5. The basic source release includes these subdirectories: - configs: example simulation configuration scripts - ext: less-common external packages needed to build gem5 - src: source code of the gem5 simulator - system: source for some optional system software for simulated systems - tests: regression tests - util: useful utility programs and files To run full-system simulations, you will need compiled system firmware (console and PALcode for Alpha), kernel binaries and one or more disk images. Please see the gem5 download page for these items at http://www.gem5.org/Download If you have questions, please send mail to gem5-users@gem5.org Enjoy using gem5 and please share your modifications and extensions.